Methods of transmitting and receiving communications signals over fiber optic networks are known. RF subcarrier division multiplexed (SDM) transmission techniques are also known. Such techniques have been applied to a variety of communications networks, including hybrid fiber optic/coaxial cable (HFC) networks for community antenna television (CATV) distribution. In fact subcarrier division multiplexing is the standard multiplexing technique used in CATV networks today.
The prior use of subcarrier division multiplexing techniques in HFC networks has generally limited the optical bandwidth utilized for the transmission to that of the coaxial cable portion of the network (e.g. 50-750 MHz). This makes sense for the analog amplitude modulation-vestigial sideband (AM-VSB) transmission of television signals where the linearity of the optical system creates an upper limit on the number of subcarrier multiplexed channels that can be transmitted with the required signal quality over the fiber optic portion of the network. Thus, such network implementations benefit from the low loss of the fiber, but fail to exploit the significant bandwidth of the fiber in the distribution of communications signals and services.
Key to the teachings of the prior art has been a belief that the bandwidth of the coaxial cable portion of the network somehow limits the laser bandwidth that can be effectively utilized to deliver services over such a hybrid network. The current industry trend is to provide allocated bandwidth services over the HFC network for applications such as internet, pay per view and telephone. These allocated bandwidth services typically have taken the form of a digital quadrature amplitude modulation (QAM) and do not require the same degree of linearity from the optical system as the AM-VSB signals have required in the past. This fact is generally known, but it is not effectively exploited in modern CATV network architectures.
The substantially relaxed linearity specifications that result from the utilization of digital QAM signals in contrast to the AM-VSB signals can impact the network architecture by two distinctly separate means. One such means is by the wavelength division multiplexing (WDM) of several laser wavelengths, each of which carries an SDM signal containing multiple QAM subcarriers. Since the linearity specifications are somewhat relaxed, these WDM signals can be transmitted over a single fiber provided that the wavelengths are separated at the hub by an optical wavelength division demultiplexing filter prior to combining one of these wavelengths with the optical carrier containing the SDM AM-VSB signals. These two wavelengths can thereafter be transmitted a reasonable distance through the fiber while maintaining the linearity performance required by the AM-VSB system.
It is important to note that in such WDM systems containing SDM digital QAM signals on the optical wavelengths, the prior art has taught the utilization of the RF bandwidth ranging from 550-750 MHz. The RF bandwidth ranging from 50-550 MHz is reserved for the SDM AM-VSB signals, and the upper bandwidth limit of 750 MHz is imposed by the transmission features of the AdI, coaxial cable (and its associated RF amplifier chain). Therefore a mere 200 MHz of optical bandwidth is utilized for the SDM QAM signals. However there is no fundamental reason why such bandwidth should be so limited. This is a restriction imposed by the coaxial cable system, not by the optical system.
The above scenario considers an analog system that carries a combination of AM-VSB signals and QAM signals on the multiplexed analog subcarriers. However, these considerations are equally pertinent to the digital communications system. The main difference is that in the case of the digital system, bandwidth is increased by time division multiplexing (TDM) of multiple digital signals, whereas in the analog system bandwidth is increased by subcarrier division multiplexing of analog subcarriers.
In the digital system the prior art has taught two distinct techniques whereby additional optical bandwidth may be utilized. One such technique comprises the utilization of multiple optical wavelengths and WDM technology. The other technique comprises increasing the bit rate of the TDM signal. Both of these methodologies require substantial upgrades to the network terminal equipment in order to enable the utilization of additional optical bandwidth.
Accordingly it is an object of this invention to provide a means for increasing the utilization of optical spectrum in the fiber optic portion of the network with the minimum impact on the network terminal equipment.
It is a further object of this invention to provide a means for distributing the additional bandwidth created by increasing the utilization of optical spectrum to different portions of the electrical (or optical) network where less total bandwidth is required.
It is a further object of this invention to provide additional utility to the network by enabling the portion of the bandwidth that is to be distributed to a given portion of the network to be remotely selectable by RF and/or optical techniques.